1. Field of the Invention
The present invention relates to a catalyst-carrying filter that is used to collect or purify particulate matter contained in exhaust gas discharged from an internal combustion engine (e.g., diesel engine) or a combustion apparatus.
2. Description of Related Art
Exhaust gas discharged from an internal combustion engine (e.g., diesel engine) or a combustion apparatus (hereinafter may be collectively referred to as “internal combustion engine or the like”) contains a large amount of particulate matter (hereinafter may be referred to as “PM”) that mainly contains soot (graphite). When particulate matter is directly discharged to the atmosphere, environmental pollution occurs. Therefore, a filter that collects particulate matter is generally provided in an exhaust gas passage connected to an internal combustion engine or the like.
For example, a honeycomb filter has been utilized for such a purpose. The honeycomb filter includes a honeycomb structure that has a plurality of cells (gas passages) partitioned by a partition wall formed of a porous ceramic having a number of pores, one open end and the other open end of the plurality of cells being alternately plugged by plugging portions. In recent years, a honeycomb filter that carries an oxidizing catalyst for promoting oxidation (combustion) of particulate matter (hereinafter may be referred to as “catalyst-carrying filter”) has been used.
For example, a honeycomb filter shown in FIGS. 6 and 7 includes a honeycomb structure that has a plurality of cells 23 (gas passages) partitioned by a partition wall 24 formed of a porous ceramic having a number of pores 25, one open end X and the other open end Y of the plurality of cells being alternately plugged by plugging sections 22. The pores formed in the inflow-side surface of the partition wall are coated with a catalyst. A gas G1 enters the pores formed in the inflow-side surface of the partition wall and is discharged to the adjacent cell through the pores formed in the outflow-side surface of the partition wall. According to this configuration, when exhaust gas is introduced into the inflow cells, particulate matter contained in the exhaust gas is collected by the partition wall when the exhaust gas passes through the partition wall. The exhaust gas from which particulate matter has been removed (i.e., purified gas) is discharged from the outflow cells. Since oxidation (combustion) of particulate matter is promoted by the oxidizing catalyst supported on the surface of the partition wall of the honeycomb filter and the inner surface of the pore formed in the partition wall, the amount of particulate matter contained in the exhaust gas can be reduced so that the exhaust gas can be effectively purified.
When using a catalyst-carrying filter formed of a porous ceramic having an average pore size that ensures that particulate matter contained in exhaust gas can be reliably collected, most of particulate matter contained in exhaust gas is deposited on the surface of the partition wall that faces the exhaust-gas-inflow cell, and does not reach the inside of the pores formed in the partition wall. Specifically, since the oxidizing catalyst supported on the inner surface of the pore formed in the partition wall does not come in contact with particulate matter; the oxidizing catalyst is not effectively utilized. Therefore, since oxidation (combustion) of particulate matter cannot be promoted sufficiently, the amount of particulate matter contained in exhaust gas cannot be reduced. As a result, particulate matter is deposited on the surface of the partition wall that faces the exhaust-gas-inflow cell within a relatively short time. Therefore, the filter must be frequently regenerated (i.e., a process that removes deposited particulate matter by back washing, heating, or the like).
The following documents disclose measures aimed at solving the above-described problems.
Japanese Utility Model Registration No. 2607898 discloses a two-layer structure in which a coating having small pores is provided on the particle inflow side. However, since the two-layer structure disclosed in Japanese Utility Model Registration No. 2607898 is not coated with a catalyst, this structure is not appropriate for purifying PM or unburned gas.
JP-B-07-106290 discloses coating a surface layer with a catalyst layer that contains only a catalyst. However, when the catalyst layer contains only a catalyst, the catalyst flows downstream toward a base material so that the effect of the catalyst layer decreases.
JP-A-2003-53121 discloses a method of producing a ceramic filter in which a porous layer is formed on an inflow-side surface. JP-A-2003-53121 aims at collecting minute dust discharged from a waste incinerator or the like using the porous layer. However, JP-A-2003-53121 is silent about a catalyst. Moreover, the method disclosed in JP-A-2003-53121 is insufficient to reduce a pressure loss due to soot (i.e., improve emission) while improving the regeneration efficiency.
Specifically, the above-mentioned documents do not disclose satisfactory measures against the above-described problems. A method that provides a fiber material on the exhaust gas inflow side has also been disclosed. However, this method suffers from poor durability, and cannot sufficiently solve the above-described problems. Therefore, a further improvement has been desired.